1. Field of the Invention
The present invention relates to a fluorescent water base ink for ink-jet recording with which the ink-jet recording is to be performed.
2. Description of the Related Art
In the ink-jet recording system, the ink is discharged, for example, from the nozzles, the slits, or the porous film to perform the recording on the recording medium including, for example, the paper, the cloth, and the film. The system for discharging the ink adopted in the ink-jet recording system includes, for example, the electrostatic attraction system in which the ink is discharged by utilizing the electrostatic attracting force, the drop-on-demand system in which the mechanical vibration or the displacement is applied to the ink by using the piezoelectric element, and the thermal ink-jet system in which bubbles are generated by heating the ink to utilize the pressure generated thereby. Ink droplets are formed by using the ink discharge system as described above to adhere a part or all of the ink droplets onto the recording medium, and thus the recording is performed.
The development is advanced for the ink which is applicable in various fields as the ink to be used for the ink-jet recording system. The ink as described above includes, for example, such a fluorescent ink that any information, which requires the security, is recorded on a recording medium while the ink is allowed to posses the fluorescent property, and the information can be read by radiating the ultraviolet light onto the ink to emit the fluorescence.
The fluorescent ink includes not only transparent inks which have no absorption in the visible region but also color inks which have the absorption in the visible region. In the case of the color fluorescent ink which has the absorption in the visible region, the information can be recognized visually even with the naked eye. The reading means can be selected depending on the contents of the information. Therefore, it is expected to widen the versatility of the ink.
However, in general, it is known for the fluorescent ink that the concentration quenching phenomenon occurs such that the fluorescence intensity is all the more decreased if the concentration of the fluorescent dye contained in the ink exceeds a certain amount, for the following reason. That is, the fluorescent dye undergoes the transition from the excited state to the more stable state without radiating the absorbed energy as the light due to the interaction of the excited fluorescent dye molecules (nonradiating process). For this reason, various trials have been made in order to enhance the fluorescence intensity within a concentration range in which the concentration quenching phenomenon is not caused. A method, in which fluorescent dyes are used in combination, is an example thereof.
An ink described in Japanese Patent Application Laid-open No. 2003-192962 is exemplified as an example in which fluorescent dyes are combined in the fluorescent ink to use the ultraviolet light as the exciting light. In this example, two types of fluorescent dyes, i.e., a first fluorescent dye and a second fluorescent dye are used. The first fluorescent dye and the second fluorescent dye have a common absorption wavelength region in the ultraviolet region. Further, the first fluorescent dye and the second fluorescent dye have a common fluorescence wavelength region in the visible region. When the two types of the fluorescent dyes are combined, then the fluorescence intensity of the second fluorescent dye is added to the fluorescence intensity of the first fluorescent dye, and the ink having the high fluorescence intensity is obtained.
However, in the case of the ink described in Japanese Patent Application Laid-open No. 2003-192962, the first fluorescent dye and the second fluorescent dye absorb the ultraviolet light in the same wavelength region, because the first fluorescent dye and the second fluorescent dye have the common exciting wavelength region in the ultraviolet region. Therefore, a problem arises such that the excitation energy, which is possessed by the ultraviolet light, is shared by the two types of the fluorescent dyes, and it is impossible to effectively utilize the fluorescence possessed by each of the fluorescent dyes.